1. Field of the Invention
This invention relates to a process and apparatus for recovering bitumen from bitumen-bearing sands and, more particularly, to a thermal process for producing synthetic crude oil, the process being particularly characterized by the absence of water or solvents in the recovery process.
2. The Prior Art
The term "tar sand" is used to refer to a consolidated mixture of bitumen (tar) and sand. Tar sand is also referred to as oil-impregnated sandstone, oil sand, and bituminous sand, among others. The latter term is generally considered to be more technically correct in that the sense of the term provides an adequate description of the mixture. The sand in tar sand is mostly alpha quartz as determined from x-ray diffraction patterns, while the bitumen or tar consists of a mixture of a variety of hydrocarbons and substituted hydrocarbons. Importantly, if properly separated from the sand component, bitumen may be used as a synthetic crude oil feed stock for the production of synthetic fuels and/or petrochemicals.
Tar sand deposits occur throughout the world, often in the same geographical areas as petroleum deposits. Significantly large, surface-available tar sand deposits have been identified and mapped in Canada, Venezuela, and the United States. The major Canadian tar sand deposit is known as the Athabasca deposit and is located in the province of Alberta, Canada. Analysis of Athabasca Tar Sand indicates an average bitumen content of approximately 12-13 percent, by weight, and a reserve estimated to be the equivalent of approximately 700 billion barrels of bitumen. To date, the Athabasca Tar Sand deposit is the only tar sand deposit in the world that is currently being commercially mined and processed for the recovery of bitumen.
A significant portion of the tar sand deposits discovered to date in the United States have been found in the State of Utah. According to a report by the Utah Geological and Mineral Survey, the State of Utah contains at least 25 billion barrels of bitumen in the form of Utah tar sands. This represents approximately 95 percent of the total mapped tar sand reserves of the United States. Although the Utah tar sand reserves appear small in comparison with the enormous potential of the Canadian tar sands, the Utah tar sand reserves represent a significant energy resource when compared to the United States crude oil proven reserves (approximately 31.3 billion barrels) and the United States crude oil production of almost 3.0 billion barrels during 1976.
Utah tar sands generally occur in six major deposits along the eastern edge of the state, and the bitumen content varies from deposit to deposit as well as within a given deposit. However, the current information available indicates that the Utah tar sand deposits average generally less than 10 percent bitumen, by weight, but have been found with a bitumen content up to 17 percent, by weight.
Typically, bitumen obtained from bitumen-bearing tar sand consists of high molecular weight molecules. These molecules are generally hydrocarbons and substituted hydrocarbons containing some nitrogen, oxygen, sulphur, etc. The bitumen recovered directly from tar sand is quite viscous. Tests have determined that bitumen from Utah tar sands is two orders of magnitude of about 100 times more viscous than bitumen obtained from Athabasca Tar Sand.
Currently, the only large-scale commercial process for the recovery of bitumen from tar sands involves Athabasca Tar Sand and utilizes a hot-water extraction technique which is conducted at temperatures just below the normal boiling point of water with substantially no change in the chemical nature of the recovered bitumen. However, this particular hot-water extraction technique is restricted to tar sands containing more than about 10 percent bitumen, by weight.
Importantly, Athabasca Tar Sand also has a relatively high moisture content of approximately 3-5 percent, by weight, connate water. It has, therefore, been postulated by certain investigators that the equilibrium structure of Athabasca Tar Sand consists of sand particles mixed with but separated from the bitumen matrix by a film of connate water. The connate water surrounds each grain of sand thereby separating the bitumen from the sand grains. Under these conditions, Athabasca Tar Sand is readily amenable to a hot-water separation technique whereby the bitumen phase is simply disengaged from the sand phase.
A more comprehensive discussion of Athabasca Tar Sand may be found in the literature including, for example, (1) E. D. Innes and J. V. D. Fear, "Canada's First Commercial Tar Sand Development," Proceedings of the Seventh World Petroleum Congress, Elsevier Publishing Co., 3, p. 633, (1967); (2) F. W. Camp, The Tar Sands of Alberta Canada, 2nd Edition, Cameron Engineering, Inc., Denver, Colo. (1974); and (3) J. Leja and C. W. Bowman, "Application of Thermodynamics to the Athabasca Tar Sands," Canadian Journal of Chemical Engineering, 46, p, 479 (1968).
Additionally, the following U.S. patents are a few of the patents that have been granted for apparatus or processes useful for obtaining bitumen for tar sands and, in some cases, specifically Athabasca Tar Sand: U.S. Pat. Nos. 1,497,607; 1,514,113; 2,871,180; 2,965,557; 3,161,581; 3,392,105; 3,553,099; 3,560,371; 3,556,980; 3,605,975; 3,784,464; 3,847,789; 3,875,046; and 3,893,907. Each of the foregoing patents deals with either the solvent or hot-water extraction of bitumen from tar sand.
Unlike Athabasca Tar Sand, Utah tar sands have been found to be so dry that no moisture content can be detected by standard analytical techniques. Accordingly, in the absence of connate water, the bitumen of Utah tar sands is directly in contact with and bonded to the surface of the sand grains. Attempts have been made to process Utah tar sands with the hot-water processes used for Athabasca Tar Sand. However, these attempts have generally proved to be unsuccessful. Additionally, it is well known that the Utah tar sand deposits occur in a region which is particularly characterized by the scarcity of excess water so that any process attempting to utilize Utah tar sands, in the absence of a special technique for dislodging the bitumen from the tar sands, should be directed to a process which requires very little, if any, water.
Canadian Pat. No. 530,920 discloses a process for recovering bituminous products from tar sands wherein tar sand containing water and chemically unaltered bitumen are introduced into a fluidized bed reactor. The bituminous matrix is partially cracked to hydrocarbon compounds having relatively short chain molecules and released from the sand particles leaving a coke residue. The coke residue is burned in a separate furnace to heat the sand residue. The heated sand is returned to the fluidized bed reactor where it supplies the required thermal energy to raise the temperature of the raw material in the fluidized reaction bed. It is, therefore, obvious that this type of thermal energy transfer through the use of recycled sand requires extensive materials handling. Additional heat may be supplied by burning a portion of the liberated gas such as hydrogen, methane, and the like in the fluidized bed.
The foregoing materials handling problem is particularly relevant in view of the substantial quantities of materials expected to be handled. For example, a process handling 5,000 pounds per minute of tar sand containing 14 percent bitumen and 1 percent water requires recycling about 12,750 pounds of hot, burnt sand per minute from the furnace to the reaction chamber to maintain the reaction temperature, see column 6, lines 26-29. It would, therefore, be an advancement in the art to provide an improved apparatus and process for recovering bituminous products from tar sands wherein (1) material handling is reduced to a minimum, (2) the solids pass through the reaction vessel in a single pass and are assisted by gravity in passage, and (3) thermal energy is transferred by heat pipes between fluidized bed reactors. Such an invention is disclosed and claimed herein.